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FIRING BISCUIT (Pottery, for artists)
CHAPTER X FIRING BISCUIT "The Pope the Cardinals, and the Princes of the World are astonished that such excellent and noble works can be made out of the earth." EXIMENUS. Fifteenth century. WHEN sufficient green shapes have been accumu- lated and are white dry, the next stage will be biscuit- ing. This process is the firing of the clay to a prim- rose or a white heat according to its fusibility. This permanently expels the water that is always present, even when dry, and converts the friable clay into a hard unalterable body. This may vary in colour from the white of kaolin to ivory, grey, buff, red, or brown, according to the composition of the clay; it may be vitreous or porous; soft like common flower pots or so hard that it will spark when struck with steel. The fire is the ultimate test of the pot and of the potter. It is indispensable to both. With but a small kiln the craftsman will begin to appreciate many things that can be learned only at the fire hole. Without a kiln he will not commence to be a potter. 93 94 CAS KILN' 5HAPL ONTU.L INSlDt&CAST JHAPL ONTO?. B.CONE5" O HEAVY POT (SMALL INSIDC) DTlLLJONJA DDLLf-nRLD- TILL Ot^TOP- F1WL3R1CK CASL SHUT IKON CASING- JECTION OF FIG. 45 The kinds of kilns usually found in schools are the gas and the oil kiln. The English gas has an arrangement of nine or twelve burners beneath the muffle. (Fig. 45.) This is a fire clay box, open at the front, set on fire bricks and cased round with fire tiles within an iron frame (see cut). There is an air space all round, except at the open end, lead- ing to the flues on top which have dampers to regulate the draught. The open end may be closed by a hinged door or bricked up with fire tiles cut to fit. The defects of this kind of kiln are too sud- den access of flame to the bottom of the muffle, causing it to split, and the impossibility of getting the front, where trials are usually placed, fired up equally 95 [with the back. An ideal muffle of this kind would !be one with flues all round, gradual access of flame ion all sides, spy holes each end, and the top to lift off, for placing. American kilns have flues in the door, and the chimney at the top is placed slightly forward, thus making it easy to fire the front up hard. They are usually fitted with two large burners, with air mixers, and a handy mica spy hole. The oil kiln differs considerably from the gas kiln. The oil is fed through tubes into fire boxes some distance below the bottom of the muffle. It burns on asbestos fibre in an iron pan to which the draught can be admitted. The flames strike the bottom of the muffle and pass up through fire clay pipes, which project into the muffle, then pass off through a twin flue regulated by dampers. With this kiln a long flue is necessary and any excess of carbon is liable to choke the pipes, and further it cannot be "sweated" up at the finish so easily as a gas kiln having a large number of burners. It is a good plan when a coarse fire clay muffle is used for glaze and biscuit to give the sides and top a sagger wash of lead and stone. This renders the muffle less liable to absorb glaze from the pots in glost firing. It also lessens the danger of small bits scaling off and sticking to the finished ware. The bottom should always be kept dusted with finely powdered flint. When cracks appear or 96 joints open, they should be stopped with a pugging of fire clay and grog. A mixture of egg silica or water glass with fine grog and quartz sand will stop small cracks. Siluma, a fire-proof cement, with equal parts of sand, answers admirably for patching. In biscuit firing the green shapes may be packed close together, with the lighter shapes on top of the stronger, but all must be J" to f " away from the sides of the muffle. Triangular pieces of biscuit, called saddles, are used to raise the shapes off the bottom, but often a fired tile, sanded and placed on a spur or saddles, gives the best foundation. Where two layers are required, small props and fire bats, perforated to let the heat through, will be necessary. These form shelves as the exigencies of the packing dictate. (Fig. 47.) For light shapes, thimbles and fired tiles will serve the purpose. (Fig. 48.) Where a shelf or prop rocks insecurely, a small wad of pugging (grog and clay) will give a steady bearing. Thrown bowls, if dried together and well fitting, may be fired to- gether, and large thrown pots may be filled with little ones. Cast shapes can be placed on top of thrown ones, but no liberty is to be taken with them. Flint should be used liberally to prevent sticking, which may happen if the biscuit be over-fired. Tiles can be fired two together in tile boxes or 97 stacked as dried. Flat ware fired in a small muffle requires very careful handling. Whenever possible, it should be placed in the centre, on a flat flint- covered fire tile or bat. One soon learns to pack a biscuit kiln, using sad- dles, spurs, stilts, thimbles, bits of tile or biscuit, and sand or flint as necessary. The thing that is a little difficult to realize at first is that built or thrown shapes, and still less tiles or modelled work, should not be hurried. Twelve hours is none too long to give to a f " tile in the biscuit kiln. Although to all appearances thoroughly dry, the least hurry gen- erates steam which will ruthlessly blow our best effects to bits. In packing, two cones or temperature indicators (Fig. 46) are placed somewhere near the middle in a position easily seen during the firing from the spy hole. These cones are made of different compositions which melt at varying temperatures. Thus if the firing point of a body is known, a cone of that degree is used and the firing continued until the cone bends. This it does soon after it assumes the colour of the surrounding muffle. To eliminate the uncertainty that is likely to be present at the first few firings it is as well to use two or even three cones, one just above and one below the correct temperature. Placed in order there is little chance then of over- or under-firing unless so 98 CONE./: ERECT 5HRUNK.ifNT. COLLAB/fD. FIG. 46 much sulphur gets into the kiln that the cones harden and refuse to turn. Calorites are sometimes used but are v not so reliable. The cones may be sloped to insure bending to right or left, as a cone bending towards the spy is deceptive. A trial piece of biscuit being placed near the spy hole, the next thing is to close the muffle. In a kiln with hinged doors the spy hole is fixed and this fact has to be taken into account. But with a bricked-up door the spy and vent may be left where it is most convenient. These orifices have plugs that fit them loosely so that if necessary they can be pulled without disturbing the clamming. This clamming or stopping is a mixture of sand, sieved dust, ground pitchers, or other infusible siftings held 99 together with a very little waste glaze and water. Where much is required, moist sand will suffice. This is plastered into the cracks that would other- wise let heat out of, or air into, the muffle, and so seals up the door. With hinged doors little stopping is required, except round the spy or vent. The crack above the door should not be clammed until the muffle begins to get warm. With this done and the two plugs out, all is now ready to light up. With an English kiln, a good middle course is as follows : For cone .01, taking 12-14 hours. See that all the burner taps are off with the main cock on one third to one half. Take the reading of the metre. Pull the air regulators right back and the dampers out nearly half. Then take out the plug of the lighting hole and insert a taper. Turn on tap number 1. When lit, withdraw the taper and turn on tap number 2. Turn down to about one half and continue until every burner is lit, making sure that each one is burning freely with a yellow flame. With a kiln having twelve burners turn out all but numbers 3, 6, and 9. With these on one third, very gently push forward the air regu- lators until a roaring noise tells that air is being admitted to the bunsen burners. The flame at this time should be blue, and the stopper should be replaced. If the flame appears at all fierce, turn the taps down a little. If turned 100 BATS Si PROPS FIG. 47 down too much, the gas lights back and will have to be turned out and relighted. The same thing happens when too much air is admitted. The burn- ers require watching until the right pressure is known. For biscuit of any thickness three hours on the three burners is not too much, the taps being grad- ually turned on to increase the length of the flame until at the end of the three hours the taps are at three fourths. At the end of the first half hour the bottom spy hole is plugged and when all steam has 101 LIGHT BAT ipped issuing from the top vent, that is stopped so and the whole clammed, leaving just a small crack as vent. When three hours are up, the regu- lators are pulled back and all burners lit at half cock. Then all but 2, 5, 7, 9, and 1 1 are turned out. Starting at the half, they are grad- ually increased to three fourths in two hours. Then numbers 1, 2 4, 5 7, 8 10, 11 are lighted in the same way. Colour will be- gin to show inside the muffle about the fifth or sixth hour, and the top dampers can come out a little, the front one more than the back, to draw the flame towards the door. When the eight burners have been on about one and one half hours the muffle will probably show a dull red inside and all danger of blowing will be passed. Two more burners at three fourths may now be put on and at the expiration of another one and one half hours the remaining two may be turned TlLf THIMBUS FIG. 48 on. If the pressure weakens and the flame shortens, the main cock should be turned on. Just above the base of each chimney is a small hole and the flame should show through these towards the finish. If the flame flaps out of the chimney tops, it is so much waste and it should be checked at the taps. The dampers can come right out towards the end, being used to sweat the flame to the front if the back appears to come up too fast. As white heat approaches, the cones should be observed occasionally. They turn the colour of their surroundings when about to topple over and as the first begins to curl the trial near the spy can be hooked out quickly and tested. When the right cone is well down, the main cock and then the burner taps are turned off. The air regulators are now pulled back and after a few minutes the dampers are closed. The metre is then checked and entered in the firing book. The kiln should be allowed to cool slowly for at least 12 hours, but the clamming at the top may come away and this will expedite the cooling with- out risk to the muffle. This time applies to a muffle of about 20" x 15" X 30" dimensions. A smaller kiln will fire up and cool in a shorter time. Gas kilns as made in America vary in the arrange- ment of their burners, but the principle is the same. They are simple to manipulate and especially handy 103 for firing " on " decoration, as they are fitted with shelves and uprights. The burners once alight, the flame is gradually increased, but where the press- ure is uncertain, it is well to keep something in reserve. Firing with oil is somewhat different. The as- bestos fibre in the pans is well saturated with kerosene, the tanks filled, and the taps turned off. A light is applied to the burners and when both are burning freely the taps are turned on to allow a thin stream of oil to flow into the pans. From the merest trickle at first, the flow should be gradually increased as the heat develops. This is observed through the mica spy hole in the door and the one above tells when the flame is reaching its maxi- mum. Should it flare over irregularly before the finish it means that the combustion is not per- fect and there is danger of clogging. The supply of oil should be reduced and the draught regulated until the flame in the combustion chamber burns clear. All soot or carbon forming in the fire box should be raked out and the oil supply checked, as it indi- cates a too liberal supply. As the oil in the tank subsides it should be refilled and the taps checked, as the increase in pressure is apt to vary the flow. The later patterns of oil kilns have several advant- ages over the kiln described. The muffle construe- 104 tion and the burner arrangements are ingenious and practical, and need little manipulation to insure even distribution of heat. The oil tanks and taps will need attention at each firing, otherwise sediment will collect and choke the even flow of paraffin oil or kerosene. With both gas or oil kilns the amount of fuel consumed should be recorded, together with the time, weather conditions, cones, and results of firing, in the " Kiln Log." IL FJ&E.BOX/ SKETCH JIG OIL KILN OF GLAZED POT/. A SPY-HOLE D POT OK JB THRlECONt/ E nOjo c TQTWnfi LEG/ F POT/ OM 5HKED JTILT. ^ .men/- G*icc JTtCJ? FIG. 49 [[Categoría:Pottery, for artists, craftsmen & teachers]